One of the main purposes of ultra high field (UHF) magnetic resonance imaging (MRI), i.e. MRI using magnetic fields B0 of 7 T or more, is to improve spatial resolution, thanks to an increased signal-to-noise ratio (SNR). However, at UHF, the Larmor frequency of protons corresponds to wavelengths of a few centimeters or tens of centimeters, i.e. comparable to the size of anatomical features such as the head. This leads to enhanced non-uniformities in the radio-frequency (B1) field, and therefore to the appearance of low-SNR zone across the images, detrimental to diagnosis. Static magnetic field inhomogeneity (ΔB0) induces similar problems.
Several tools have been proposed to solve this problem, including adiabatic pulses, RF shimming and parallel transmission.
Adiabatic pulses (see e.g. [1]) are amplitude and frequency modulated pulses, which effectively reduce B1 inhomogeneity as long as the field amplitude is above a certain threshold and that the pulse duration is sufficiently long. They have been a huge success at low field. However, their use is limited at ultra-high field because they necessitate long durations, large peak powers and large energies, thus large specific absorption rates (SAR, indicative of patient safety), which is problematic for in vivo studies.
RF shimming (see e.g. [2]) uses a plurality of radio-frequency (RF) coil elements, transmitting the same RF pulse with different complex weights, to homogenize the B1 field. As explained in [3], several sets of RF-shimming parameters—optimal for different categories of patient—may be pre-computed; during clinical activity, the best suited one of these sets of parameters is chosen for each given patient. This technique is attractive due to its simplicity, but in practice it only gives satisfactory results at UHF in very specific cases.
Transmit-sense parallel transmission, or “pTx”, (see e.g. [4]) is a much more versatile technique, and so far has proved to be almost indispensable to tackle the RF and static field inhomogeneity problem at UHF for SAR-demanding sequences. This technique uses several independently-driven and spatially separated transmission channels, including respective coil elements placed around the subject, to simultaneously transmit RF pulses having different temporal waveforms (usually defined by complex envelopes). Moreover, a magnetic field gradient waveform, which defines a predetermined trajectory in k-space, is played at the same time as the RF pulses and/or between them. The complex envelopes of the RF pulses are designed to maximize the homogeneity of the nuclear spin excitation (i.e. flip angle) across a volume of interest, instead of homogenizing the field itself, as in RF shimming (in some particular case, the aim will be to obtain a flip-angle spatial distribution as close as possible to non-homogeneous target). The optimization of the RF pulses is usually carried out under local SAR constraints; see e.g. [5] and [6]. Document [7] discloses a method for simplifying the computational burden induced by the consideration of local SAR constraints.
Despite its effectiveness in homogenizing the nuclear spin excitation, parallel transmission has remained essentially a research instrument, and its use in clinical practice is still negligible twelve years after its introduction. This is mainly due to the fact that its implementation is difficult and laborious, as it starts with the acquisition of B1 and ΔB0 maps for each patient (and, concerning B1, for each individual transmission channel), and follows with a numerical calculation performed on the fly (online)—when the subject is in the MRI scanner—to optimize the RF pulse depending on the measured B1 and ΔB0 maps.
Even using the most effective available approaches, acquiring B1 and ΔB0 maps ([7], [8]) takes at least 5-7 minutes, to which one must add the time required to perform the calculations for optimizing the pulses. While this duration is acceptable for research purposes, this may not be the case for the clinician who can hardly afford more than about 10 minutes to proceed with the clinically relevant acquisitions. Moreover, in addition to a lack of comfort, the patient can have a condition which hardly prevents him from moving (i.e. Parkinson's disease). Thus, the application field of pTx is limited today by the extra-time it requires.